With the FTSE 100 index recovering 15 per cent from the March 17 low point, and showing no signs of slowing to date, investors who follow the 'sell in May' theory must be more than a little nervous.

In practice I suspect very few investors sell up their entire portfolios each year on the strength of an old saying, even though history confirms share prices do generally drift lower during the quieter summer months. Dealing costs and capital gains tax will put many off and the vast majority of shareholders are invested for the longer term which is the best way to maximise returns from equity investment.

Returning to the recent rally in the FTSE 100 index, this performance masks a very mixed picture when you look at the individual sectors and companies that make up the index.

For example, share prices across the bank, construction and real estate sectors are generally lower today than they were a month ago. However, these falls have been more than cancelled out by rising share prices in the heavyweight mining and oil and gas sectors. These sectors account for around a third of the FTSE 100 index and have been the driving force behind the recent strength in the leading index.

Equity markets have also reacted positively to the lack of bad news coming out of the bank sector. Having said that, yesterday's gloomy statement from the Alliance & Leicester is a timely reminder that no news is (not necessarily) good news.

This mixed picture will continue this week with updates due tomorrow from Barclays and Currys owner DSG; not to mention full-year results from British Airways on Friday.

The banks and retailers are facing difficult trading conditions in the wake of the credit crunch and British Airways has of course had its own problems of late. While BA is expected to announce a solid set of results on Friday, attention is likely to focus on the disastrous opening of Heathrow's Terminal 5 and the high oil price.

On the bright side, First Choice owner Tui Travel and Premier Foods pleased shareholders with positive statements earlier this week.

Investor sentiment should be buoyed further by tomorrow's annual results from National Grid. Earnings from the owner of the UK's electricity grid and gas pipeline are fairly predictable so the odds of a disappointment from National Grid are slim.

Finally, copper and zinc miner Vedanta Resources will also announce annual results tomorrow where strong commodity demand from China and India should ensure a solid set of results.

David Evans